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Why are Polycrystalline Solar Cells so Popular?

Polycrystalline (sometimes also called multicrystalline) solar panels are the most common because they are often the least expensive. They are the middle choice in the marketplace ... almost as good as single cell monocrystalline silicon panels but generally with a better efficiency than thin film solar panels.

Polycrystalline cells can be recognized by a visible grain, a “metal flake effect”. The solar cells are generally square in shape, and may have a surface that looks somewhat like a mosaic. That’s because of all the different crystals that make up the module.

How Polycrystalline Cells Are Made

The reason polycrystalline solar panels are less expensive than monocrystalline solar panels, is because of the way the silicon is made. Basically, the molten silicon is poured into a cast instead of being made into a single crystal.

This material can be synthesized easily by allowing liquid silicon to cool using a seed crystal of the desired crystal structure. Additionally, other methods for crystallizing amorphous silicon to form polysilicon exist such as high temperature chemical vapor deposition (CVD).

In the cast process, silicon pieces are melted in a ceramic crucible and then formed in a graphite mold to form an ingot. As the molten silicon is cooling a seed crystal of the desired crystal structure is introduced to facilitate formation.

Although molding and using multiple silicon cells requires less silicon and reduces the manufacturing costs, it also reduces the efficiency of the solar panels.

Efficiency

Generally speaking, polycrystalline panels have an efficiency that is about 70% to 80% of a comparable monocrystalline solar panel. The most efficient polycrystalline panels are built by Mitsubishi Electric Corporation. In February 2010, Mitsubishi set two world records for photoelectric conversion efficiency in polycrystalline silicon photovoltaic (PV) cells, which was achieved by reducing resistive loss in the cells. The conversion efficiency rates have been confirmed by the National Institute of Advanced Industrial Science and Technology (AIST), in Japan.

Another one of the world records, which Mitsubishi Electric has now renewed for the third consecutive year, is a 19.3-percent efficiency rating for photoelectric conversion of a practically-sized polycrystalline silicon PV cell of 100 squared centimeters or larger, with the PV cell measuring approximately 15cm x 15cm x 200 micrometers. The rating is 0.2 points higher than the company's previous record of 19.1 percent.

The second world record, achieved with the same technologies in an ultra-thin polycrystalline silicon PV cell measuring approximately 15cm x 15cm x 100 micrometers, is an efficiency rating of 18.1 percent, a 0.7-point improvement over the company's previous record of 17.4 percent.

Currently the solar industry is investing lots of money in research and development to find ways to increase manufacturing costs and boost overall efficiency of the solar modules. As you can see from the work done by Mitsubishi, these improvements are primarily incremental in nature and are more on the manufacturing side than on the efficiency side.

Benefits of Polycrystalline Solar Panels

1. Lower Per Panel Costs

are much simpler to produce, and cost far less to manufacture. This makes them much less expensive for buyers, especially those with small to medium sized roofs.

2. Durability and Longevity

The durability and longevity are comparable to their monocrystalline cousins – namely at least 25 years. Polycrystalline solar panel modules could put solar power into the hands of people who could not afford the polycrystalline cells.

3. Environmental Enhancements

Besides being able to produce energy from the sun and thus help reduce greenhouse gases and related environmental problems of extracting fossil fuels (e.g., the BP oil spill, coal mining accidents, geo-political resource wars, etc.), some polycrystalline solar panel manufacturers (e.g., Mitsubishi) go the extra mile by inventing new technologies that eliminate expensive soldering (which also contains lead) making these panels even more environmentally friendly.

4. Lower Electric Bills

Any solar system can and probably will result in a lower electricity bill. Even though the amount of electricity produced from a polycrystalline solar panel is less than from a monocrystalline panel – so are the costs … so you have to fine tune your analysis to see which one has the better payback over the time frame of your analysis (e.g., 20 years in Europe – which is usually the time period of the Feed in Tariffs).

Disadvantages of Polycrystalline Solar Panels

1. Lower Efficiency

Polycrystalline solar modules are less efficient than those made from a single crystal.

2. Fragile

Polycrystalline solar panels are somewhat fragile, and can be broken if hit by a falling branch or reasonably heavy object flying through a strong wind.

3. Competitive

There is strong price competition between polycrystalline manufacturers, and this can be both a good thing (in that it tends to keep prices low) or a bad thing (some manufacturers may not be able to withstand the competition and won't be around to honor their product or performance warranties).

Current Market Overview

The current market for solar PV is dominated by crystalline silicon (c-Si) solar panels (around 80%), and c-Si solar technology is expected to continue to dominate in the residential and commercial rooftop markets due to higher efficiency and rapidly reducing costs.

There has been a 40% price reduction since the middle of 2009, largely as a result of the improved supply of polysilicon, which is the basis of c-Si-based panels. When supply was constrained by limited production of polysilicon, the price reached over $300/kg. Now, the cost has fallen to below $100/kg and supplies are readily available for mass production — driving a continuing decline in panel prices.

Lower cost c-Si panels support a key goal for solar known as grid parity, where it costs the same to generate power on their rooftops as it does to buy it from the grid. This point has already been reached during the peak demand period. According to the European Photovoltaic Technology Platform group, solar PV is expected to reach grid parity in most of Europe over the next 10 years.

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